This invention relates to a device for amplifying transmission power in a radio telephone system of small size, such as, a land mobile radio telephone system or a portable radio telephone system.
With the recent research and development for the radio telephone system of the kind described above, the telephone set of the land mobile radio telephone system or of the portable radio telephone system is being more and more reduced in size and weight. Points of great importance to be considered to attain the desired reduction in the size and weight of the telephone set in such a telephone system are how to reduce the size of its transmission power amplifier part and how to improve the efficiency of transmission power amplification. For this purpose, it is the recent tendency to employ a GaAs power amplifier in which a GaAs FET is used as an amplifier element. In this GaAs power amplifier, an output signal transmission line is connected to the output circuit of the GaAs FET, and a predetermined gate voltage is applied to the gate of the GaAs FET so as to amplify an input signal applied to the GaAs FET. In this case, the gate voltage applied to the gate of the GaAs FET is a minus voltage maintained constant regardless of the value of the frequency of the signal to be amplified.
Therefore, the power amplification characteristic peculiar to the GaAs power amplifier is not modified at all, and its power amplification-frequency characteristic appears directly as the transmission power amplification efficiency .eta. of the GaAs power amplifier. FIG. 1 shows the above frequency characteristic peculiar to the GaAs power amplifier, and, in FIG. 1, the horizontal axis represents the frequency of the transmission band, and the vertical axis represents the efficiency .eta. of transmission power amplification. It will be seen in FIG. 1 that the relation between the transmission power amplification efficiency .eta. and the frequency of the transmission band is not constant, and the transmission power amplification efficiency .eta. changes or decreases with the increase in the frequency of the transmission band.